There are fewer business meetings on Fridays. Hence the they can reduce the number of LCY flights, at least on the morning. From EDI one a/c does a PMI rotation early on a Friday, although it returns to LCY duties for the Friday afternoon weekend traffic. (Actually it's the only BACF flight on the public timetable - the other holiday rotations are all charters) IAG are simply making best use of their assets.